Polyester-based aqueous coating composition

ABSTRACT

Polyester-based aqueous coating compositions comprising a mixture of a carboxyl-functional polyester resin (A), a water-insoluble epoxy resin (B), and a hydrophobic solvent (C), the mixture being neutralized with neutralizer (D) and dispersed or dissolved in water. The compositions have excellent film forming and processing characteristics, and are particularly useful in coating the interior surface of cans and as automobile coatings.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to crosslinkable aqueous coatingcompositions which exhibit excellent film-forming and curingcharacteristics, in terms of processability and safety. The compositionscan be used for coating the interior surfaces of cans. Moreover, thesecrosslinkable aqueous coating compositions are capable of forming filmshaving excellent hardness, and accordingly can be used as primers orfinish coats for automotive or industrial applications.

[0003] 2. Description of the Prior Art

[0004] In the past, aqueous coating compositions have been made from thereaction product of a high acid value acrylic resin and a high molecularweight bisphenol A based epoxy resin. These compositions have been usedfor coating the interior surface of cans. However, recently questionshave been raised about bisphenol A, and effort has been directed tocoating compositions in which bisphenol A does not elute.

[0005] Polyester resins have previously been proposed for use as a baseresin in aqueous coatings. Japanese Laid-Open Patent Application No.61-37811 discloses a thermosetting resin composition prepared byemulsifying a water-soluble polyester resin and an epoxy resin. Thecomposition contains hydrophilic solvent for polyester resin, andexhibits increased dispersion power. However, the film forming abilityand processability for coating the interior surface of cans isinadequate. In addition, attempts to improve processability often resultin reduction of the stability of an emulsion of this type.

[0006] Compositions for coating substrates other than cans have beenprepared from polyester resin obtained by using hydrophilic materialssuch as those having sulfonic acid metal salt, polyalkylene glycol,aliphatic dicarboxylic acid and the like. These compositions exhibitgood solubility and dispersibility in water. However, the waterresistance of the resulting coating was not satisfactory. JapaneseLaid-Open Patent Application No. 57-40525 and No. 61-37811 disclose atechnique to raise water dispersion capacity in which a hydrophilicsolvent is used in the polyester resin having a content of hydrophilicmaterial reduced. However, the hardness and physical properties of theresulting coating were inadequate.

[0007] Aqueous coating compositions have also been proposed containing apolyester resin having hydroxyl group and carboxyl group, an alicyclicepoxy resin and a quaternary ammonium compound. These are described inJapanese Laid-Open Patent Application No. 4-359075. These compositionsprovided good storage stability and film properties. Continuing efforthas been directed toward further improvement of stain resistance andscratch resistance in the final finish. In addition, it has beendifficult to provide a good balance between a high hardness in thecoating s made using the polyester resin and emulsion stability afterdispersion in water.

SUMMARY OF THE INVENTION

[0008] The present invention is provides polyester-based aqueous coatingcompositions which exhibit good emulsion stability after dispersing inwater, excellent processability, film forming ability, and costperformance when used in coating the interior surface of cans.

[0009] The invention also provides polyester-based aqueous coatingcompositions which exhibit good emulsion stability after dispersing inwater, excellent curability, film forming ability and film hardness whenused as either an undercoating or finish coating on automobile bodies orindustrial products.

[0010] Specifically, the present invention provides a polyester-basedaqueous coating composition comprising a mixture of carboxyl-functionalpolyester resin (A) which is a condensation product of at least onepolyalcohol of which ethylene glycol comprises at least about 60 mol %based on the total polyalcohol component and at least one polybasic acidof which polyvalent aromatic carboxylic acid comprises at least about 80mol % based on the total polybasic acid component, and which has anumber average molecular weight of about from 1,000 to 20,000 and anacid value of about from 10 to 170 mgKOH/g;

DETAILED DESCRIPTION OF THE INVENTION

[0011] The carboxyl-functional polyester resin (A) used in the presentinvention is a condensation product of a polyalcohol componentcomprising at least about 60 mol ethylene glycol, based on the totalpolyalcohol components, and a polybasic acid component comprising atleast about 80 mol %, based on the total polybasic acid components, of apolyvalent aromatic carboxylic acid, and contains carboxyl groups in theresin. The polyester resin has a number average molecular weight ofabout from 1,000 to 20,000 and an acid value of about form 10 to 170mgKOH/g.

[0012] The polyester resin can be prepared by any of the followingmethods (1), (2), or (3). Method (1) involves esterifying polybasic acidcomponents and polyalcohol components using an excess of the former overthe latter. Method (2) involves reacting an acid anhydride with apolyester polyol which is obtained by reaction of polybasic acidcomponents and polyalcohol components with a molar excess of thepolyalcohol components. Method (3) involves a first step of preparing ahydroxyl-functional polyester by alcoholysis of a high molecular weightpolyester such as polyethylene terephthalate or polybutyleneterephthalate; then esterifying the resulting hydroxyl-functionalpolyester with polybasic acid components, and if necessary, polyalcoholcomponents, under conditions in which the polybasic acid components arein molar over the hydroxyl-functional components.

[0013] Examples of polybasic acids which can be used in the presentinvention, mainly in the preparation of the polyester resin, includedicarboxylic acids such as phthalic acid, phthalic anhydride,isophthalic acid, terephthalic acid, tetrahydrophthalic acid,tetrahydrophthalic anhydride, hexahydrophthalic acid, hexahydrophthalicanhydride, hexahydroisophthalic acid, hexahydroterephthalic acid,4-carbonylhexahydrophthalic acid, 4-carbonylhexahydrophthalic anhydride,3-carbonyltetrahydrophthalic acid, 3-carbonyltetrahydrophthalicanhydride, succinic acid, succinic anhydride, fumaric acid, adipic acid,azelaic acid, sebacic acid, maleic acid, maleic anhydride and loweralkyl esters of these dicarboxylic acids. In addition to thesedicarboxylic acids, monobasic acids such as benzoic acid, crotonic acid,p-t-butyl benzoic acid and other various fatty acids can be used.Further, trivalent or higher polycarboxylic acids such as trimelliticacid, trimellitic anhydride, methylcyclohexene tri-carboxylic acid,pyromellitic acid, pyromellitic anhydride and butane tricarboxylic acidcan also be used. Among these polybasic acids, it is important that thepolyvalent aromatic carboxylic acid comprise at least about 80 mol %,preferably more than 90 mol %, based on the total polybasic acidcomponents. This high concentration of the polyvalent aromaticcarboxylic acid has been found to contribute good hydrolysis stabilityresistance.

[0014] Examples of the polyalcohols which can be used in the presentinvention, mainly in the preparation of the polyester resin, includedihydric alcohols including aliphatic glycols such as ethylene glycol,diethylene glycol, triethylene glycol, propylene glycol, 1,2 butanediol,1,3-butanediol, 1,4-butanediol, neopentyl glycol, 2-methylpropane diol,3-methylpentane diol, 1,4-hexane diol, 1,6-hexane diol, 1,5-pentanediol, 1, 9 -nonane diol, diethyl pentane diol, and 2 - butyl ethylpropanediol; alicyclic glycols such as cyclohexane dimethanol and spiroglycol; aromatic glycols such as ethylene oxide or propylene oxideaddition product of bisphenol compound; polyether polyol such aspolyethylene glycol, polypropylene glycol, polybutylene glycol;polyurethane polyol obtained by the reaction of glycol andpolyisocyanate compound. In addition to these dihydric alcohols,trivalent or higher polyhydric alcohol such as glycerin,trimethylolethane, trimethylolpropane, pentaerythritol and the like canbe used. These polyalcohols can be used singly or as a mixture of two ormore. Further, a glycol including ester functionality in the molecularstructure such as bishydroxyethyl terephthalate can be used. Among thesepolyalcohols, ethylene glycol should comprise at least about 60 mol %,preferably more than about 70 mol %, based on the total polyalcoholcomponents. This high concentration of ethylene glycol provides goodfilm hardness and, in the coating to cans, flavor-retaining ability.Further, it is preferred that butylene glycol be used in combinationwith ethylene glycol. This combination results in excellent physicalproperties in the resulting film, such as processability. It is assumedthat molar quantity of glycol including ester function in molecular addswith mole ratio of minimum unit monomer. In a bishydroxyethylterephthalate, for example, it calculates as terephthalic acid 1 mol andethylene glycol 2 mol.

[0015] The condensation or transesterification of both above-mentionedcomponents can be performed by known methods to form the polyester resin(A).

[0016] In the first method, the polyester resin (A) can be obtained bymeans of direct condensation or transesterification of the polybasicacid component and the polyalcohol component, with the polybasic acidbeing in molar excess over the polyalcohol. In that case, to provide thebest processability, it is preferable that the amount of trivalent orhigher component based on a total of the polybasic acid component andthe polyalcohol component be less than about 12 mol %, preferably lessthan 7 mol %.

[0017] In the second method for preparing polyester resin (A), thepolyester polyol can be prepared by means of the direct condensation ortransesterification of the polybasic acid component and the polyalcoholcomponent on the condition that the polyalcohol is in moar excess of theacid. In the case of the preparation of the polyester polyol, to providethe best processability, it is preferable that the amount of thetrivalent or higher component based on the total of the polybasic acidcomponent and the polyalcohol component be less than about 12 mol %,preferably less than 7 mol %. The polyester resin (A) can be obtained bythe reaction of an acid anhydride to the polyester polyol. Examples ofacid anhydrides which can be used include phthalic anhydride, succinicanhydride, maleic anhydride, hexahydro phthalic anhydride, andtrimellitic anhydride.

[0018] In the third method, polyester resin (A), the polyester resin (A)can be obtained by first preparing a hydroxyl-functional polyester byalcoholysis of a high molecular weight polyester as a starting materialand a second step of directly esterifiying or transesterifying thehydroxyl-functional polyester thus obtained, polybasic acid componentand, if necessary, the polyalcohol component, provided that thepolybasic acid component is in molar excess over the hydroxyl-functionalcomponents. In this case, to provide the best processability, it ispreferable that the amount of the trivalent or higher component based onthe total of the polybasic acid component and the polyalcohol componentbe less than about 12 mol %, preferably less than 7 mol %.

[0019] In the methods (1), (2), and (3) above, the reaction of thedirect condensation or transesterification can be promoted by operatingunder pressure or reduced pressure, or flowing inert gas. Further, anorganometallic catalyst such as di-n-butyl tin oxide can be used as acondensation catalyst. Of these three methods, method (1) is preferredon the basis of ease of control of molecular weight and acid value.

[0020] The carboxyl-functional polyester resin (A) has a number averagemolecular weight of about from 1,000 to 20,000, preferably about from2,000 to 10,000, and an acid value of about from 10 to 170 mg KOH/g,preferably about from 25 to 75 mg KOH/g. These characteristics providegood processability, flavor-retaining ability, curability and stability.The ratio of the acid and alcohol components, and the specific kind ofeach component can be adjusted, according to factors known in the art,so that a polyester resin having above property values is provided. Inaddition, for good dispersibility and curability, thecarboxyl-functional polyester resin (A) should have a hydroxyl value ofabout 10 mg KOH/g or less, preferably 5 mg KOH/g or less, and have aglass transition temperature of about 0° C. or more, and preferablyabout from 20 to 80° C.

[0021] The water-insoluble epoxy resin (B) is used as a curing agent andhas at least two epoxy groups in a molecule. Examples of the epoxy resinwhich can be used include glycidyl ether compounds such as2-glycidylphenyl glycidyl ether, 2,6-diglycidyphenyl ether; glycidylester of aliphatic alcohol and alicyclic alcohol; bisphenol based epoxyresin which is prepared by reaction of epichlorohydrin with bisphenolcompound such as bisphenol F, bisphenol A, and 1,1 -bis (4hydroxyphenyl)ethane; Novolac based epoxy resin such as phenol Novolac type epoxyresin, creosol Novolac type epoxy resin; epoxidized polybutadiene;alicyclic epoxy-functional resin such as alicyclic epoxyfunctionalcompound (e.g., 3, 4-epoxycyclohexylmethyl -3,4-epoxycyclohexanecarboxylate), alicyclic epoxy-functional copolymer which is obtained bycopolymerizing a polymerizable unsaturated monomer having an alicyclicepoxy-group (e.g., 3,4-epoxy cyclohexyl (meta)acrylate) with otherpolymerizable unsaturated monomers; glycidylfunctional acrylic resinwhich is obtained to use a polymerizable unsaturated monomer having aglycidyl group (e.g., glycidyl (meta)acrylate, glycidyl allyl ether). Itis preferable that an epoxy resin which does not include free bisphenolA be used for coating the interior surface of cans. Further, the Novolacbased epoxy resin is preferred from point of food hygiene and filmproperties. When free bisphenol A is completely removed by purification,even in coating the interior surface of cans, the bisphenol A type epoxyresin prepared by the reaction with bisphenol A and epichlorohydrin canbe employed. On the other hand, the alicyclic epoxy-functional resin orthe glycidyl-functional acrylic resin is preferred for use in finishcoatings.

[0022] The epoxy resin (B) can be compounded in the manner that anequivalent ratio of the carboxyl group of the polyester resin (A) andthe epoxy group of the epoxy resin (B) is in the range of about from1/0.3 to 1/1.5, preferably about from 1/0.5 to 1/1.0.

[0023] The hydrophobic solvent (C) works as a viscosity controller inmixing and dispersing, an inhibitor of crystallization of resin, and afilm forming auxiliary for use in formation of continuous film. Examplesof the hydrophobic solvent (C) which can be used include aromatichydrocarbons such as toluene, and xylene; ketones such as butanone, andmethyl isobutyl ketone; cyclic ketones such as cyclohexanone, andisophorone; alcohol ethers such as ethylene glycol monohexyl ether,ethylene glycol monophenyl ether, and propylene glycol phenyl ether;alcohols such as 2-ethylhexyl ethyl alcohol, butyl alcohol, hexylalcohol, and benzyl alcohol; petroleum solvents such as “Swasol 1500”(product of Cosmo Oil Co., Ltd.), “Solvesso 150” (product of EtsusoSekiyu KK); “Texanol” (ester alcohol, product of Eastman Chemical JapanLtd.), “Kyowanol M” (product of Kyowa Hakko Kogyo Co., Ltd.).

[0024] These solvents can be used alone, or as a mixture of two or morethereof. Of these, Cyclohexanone is particularly preferred.

[0025] The amount of hydrophobic solvent (C) should be present inquantities of about from 1 to 200 parts by weight per 100 parts byweight of the total resin solid. If the amount is less than 1 part byweight, it results in poor film forming ability and smoothness ofcoating film, while amounts higher than 200 parts by weight causesinstability on emulsifying. When the hydrophobic solvent (C) is used inquantities higher than this range, for example, to facilitate tepreparation of a mixture before water dispersion, it can be adjusted tothe range indicated above by conventional methods such as azeotropy orreduced pressure in the emulsion after water dispersion.

[0026] A water miscible organic solvent can be admixed with thehydrophobic solvent (C) before water dispersion, if necessary. Watermiscible organic solvents which can be used include, for example, butylcellosolve, monomethyl carbitol, dimethyl carbitol, monoethyl carbitol,propylene glycol monomethyl ether, acetone, methanol, ethanol, andisopropanol. It is desirable that the amount of water miscible organicsolvent is less than about 40 % by weight, preferably 20 % by weight,based on the total solvent content.

[0027] In the present invention, the mixture of the carboxyl-functionalpolyester resin (A), the water-insoluble epoxy resin (B) and thehydrophobic solvent (C) is neutralized with neutralizer (D) anddispersed into water.

[0028] A basic compound such as an amine or ammonia can be used forneutralizer (D).

[0029] Examples of the amines which can be used include alkylamines suchas trimethylamine, triethylamine, and tributyl amine; alkanolamines suchas dimethylethanolamine, methyldiethanolamine, diethanolamine, andaminomethyl propanol; cyclic amines such as morpholine, alkylmorpholine, methylpiperazine, and ethyl piperazine. The triethylamineand dimethylethanolamine are preferred among these. The neutralizationdegree is generally in the range of about from 0. 2 to 1 -0 equivalent,preferably about from 0.1 to 2. 0 equivalent, per one equivalent of thecarboxyl group in the resin (A).

[0030] The mixture including carboxyl-functional polyester resin (A) isneutralized and dispersed into water by ordinary techniques. Forexample, the mixture including the carboxyl-functional polyester resin(A) can be doped by degrees, with agitating, in aqueous mediumcontaining the neutralizer (D). In another method, the mixture includingthe carboxyl-functional polyester resin (A) is neutralized with basiccompound, and, while agitating, the resulting neutralized mixture can bepoured into water, or water can be poured into the neutralized mixture.Further, the solvent content in coating can be adjusted by removingsolvent with water under reduced pressure.

[0031] The aqueous coating compositions of the present invention canoptionally include curing catalysts, antifoaming agents, lubricants,reforming resins, pigments, flocculation inhibitors, leveling agents,rheology control agents, odorants, and cissing inhibitors oranti-cratering agents. The solvents above mentioned can be combined as asurfactant after water dispersion.

[0032] The curing catalyst promotes the reaction with thecarboxyl-functional polyester resin (A) and the water-insoluble epoxyresin (B). The curing catalysts include, for example, water-solublequaternary ammonium salts such as choline chloride; nicotinamide,organic metal carboxylate, imidazole compound, and metal chelatecompound. The amount of the curing catalyst used is generally in therange of about from 0.05 to 3 parts by weight per 100 parts by weight ofthe total solid of the resin (A) and resin (B).

[0033] The aqueous coating compositions of the present invention can beapplied to a variety of substrates.

[0034] Examples of the substrates include treated or nontreated metalplates such as aluminum plate, steel plate, and tin plate; platesobtained so that a primer such as epoxy-type or vinyl-type is applied tothese metal plates; polyethylene terephthalate (PET) sheet; cans orbottles processed by using these metal plates and PET; and those bearinga primer to facilitate electrodeposition. Known coating techniques canbe used, such as roll coating, spray coating, dip coating, andelectrodeposition coating.

[0035] The present invention is explained more fully in the followingExamples and Comparative Examples, in which parts and percentages areall by weight.

Preparation of Carboxyl-functional Polyester Resin Preparation Example 1

[0036] A stainless flask equipped with a stirrer, heater, thermometer,nitrogen gas inlet tube, fractionating unit and distillant storage wascharged with 8.3 parts of terephthalic acid, 91.3 parts of isophthalicacid, 9.7 parts of trimellitic anhydride, 88.9 parts of bishydroxyethylterephthalate, 26 parts of neopentyl glycol, and 0.1 part of dibutyl tindioxide per 100 parts of total of above materials as catalyst. Themixture was heated to 240° C. with agitation under nitrogen. After thetemperature was raised to 240° C. while distilling off condensed water,the temperature was kept at 240° C. and the reaction was allowed toproceed. When distillation water stopped at one or two hours, xylene wasadded due to promote the reaction. The polycondensation reaction wascontinued until an acid value of 45 was attained. Thecarboxyl-functional polyester resin (A-1) thus obtained had a numberaverage molecular weight of 3,000.

Preparation Examples 2 to 8

[0037] The carboxyl-functional polyester resins (A-2) to (A-8) wereprepared in the same manner as in Preparation Example 1, except that theformulation of polyester materials was charged as shown in Table 1.Table 1 also shows acid value and number average molecular weight ofeach polyester resin (A-2) to (A-8). TABLE 1 Preparation Example 1 2 3 45 Polyester resin A-1 A-2 A-3 A-4 A-5 Terephthalic Acid 8.3 16.6 16.6Isophthalic acid 91.3 49.8 99.6 74.7 41.5 Adipic acid 29.2 29.2 43.8Trimellitic anhydride 9.7 19.4 19.4 9.7 9.7 Benzoic acid 6.1Bishydroxyethyl 88.9 101.6 76.2 50.8 76.2 terephthalate Ethylene glycol24.2 6.0 Neopentyl glycol 26 58.2 26 1,4-butandiol 13.5 Acid value 45 6550 40 45 Number average 3000 2000 2500 3500 3000 molecular weightPreparation Example 6 7 8 Polyester resin A-6 A-7 A-8 Terephthalic acid8.3 Isophthalic acid 66.4 91.3 58.1 Adipic acid 14.6 87.6 Trimelliticanhydride 19.4 9.7 9.7 Benzoic acid Bishydroxyethyl terephthalate 101.688.9 Ethylene glycol 43.4 Neopentyl glycol 27 26 1,4-butandiol 13.5 Acidvalue 65 56 45 Number average molecular weight 2500 2500 3000

Production of Aqueous Coating Composition Example 1

[0038] A reaction vessel was charged with the polyester resin (A-1)obtained in Preparation Example 1 and cooled until 1 50° C. 50 parts ofcyclohexanone per 100 parts of the resin was added and the mixture wascooled to 100° C. When the temperature was 100° C., 10 parts of“ECN1299”(manufactured by Asahi Kasei Epoxy Co., Ltd., creosol Novolacphenol epoxy resin) as a curing agent was added and the mixture wasdissolved in uniformity. Subsequently, 5 parts of dimethylethanolaminewas added, and the mixture was neutralized and diluted with 275 parts ofdeionized water to obtain a transparent or opaque white water dispersionhaving a solid content of 25 %. The water dispersion had good emulsionappearance without precipitating after the storage at 20° C. for onemonth.

[0039] The resulting water dispersion was mixed with 0.25 parts ofcholine chloride as a curing catalyst and 10 parts of isopropanol asurfactant, followed by stirring to obtain an aqueous coatingcomposition.

Examples 2 to 4 and Comparative Examples 1 to 4

[0040] The general procedure of Example 1 was repeated, except that thecompositions shown in Table 2 were used, to obtain aqueous coatingcompositions having a solids content of 25%. But, in the composition ofComparative Example 2, water dispersion could not be performed, andtherefore the aqueous coating composition was not obtained.

Performance Test

[0041] The aqueous coating compositions obtained Examples andComparative Examples were applied to aluminum sheets with a bar coaterto form coating films having a thickness of 15 μm (when dried), followedby baking at 230° C. for 10 minutes. Each coating film was subjected tothe following performance tests; The results are summarized in Table 3.

[0042] (* 1) Film surface condition: The surface of the film wasvisually evaluated according to the following criteria.

[0043] A: Excellent smoothness with no foaming over the entire film

[0044] B: Slight unevenness with small foam all over the film

[0045] C: Slight unevenness with large foam all over the film

[0046] (*2) Gel fraction ratio: The aqueous coating composition wasapplied to a weight of W1 of tin plate to obtain a coated plate having aweight of W2. A flask equipped with reflux condenser is charged with thecoated plate having a weight of W2 and methyl ethyl ketone so that thecoating area to the amount of methyl ethyl ketone may be 100 cm2 to 100cc, followed by heating to reflux for 1 hour. Thereafter the coatedplate was picked up and dried at 120° C. for 30 minutes.

[0047] The coated plate was cooled down until room temperature and aweight of W3 of the coated plate was measured. The gel fraction ratio(%) was pursued by means of the following formula.

Gel fraction ratio (%){(W3-W1) / (W2-W1) }* 100

[0048] (*3) Processability: A coated plate was folded into two equalparts in such a way that the film was outside. A 1-kg iron load wasdropped on the bent portion of the coated plate from a height of 50 cm.A length of crack of the bent portion of the coated plate was measuredand evaluated according to the following criteria.

[0049] A: Less than 5mm

[0050] B: 5mm or more and less than 20mm

[0051] C: 2mm or more

[0052] (*4) Water resistance: A coated plate was treated at 125° C. for35 minutes in an autoclave followed by dipping into water to evaluatevisually a degree of blushing of the film according to the followingcriteria.

[0053] A: No blushing

[0054] B: Slight degree of blushing

[0055] C: Remarkable degree of blushing

[0056] (*5) Adhesion: Squares were formed by effecting 11 cutsrespectively in length and width at about 1.5 mm intervals on a film ofa test plate by using a knife. An adhesive cellophane tape having awidth of 24 mm was adhered to the squares, followed by strongly peelingthe tape to evaluate the adhesion properties of the squares according tothe following criteria.

[0057] A: No peeling

[0058] B: Slight degree of peeling

[0059] C: Remarkable degree of peeling

[0060] (*6) Adhesion after water resistance test: A coated plate wastreated at 125 V for 35 10 minutes in an autoclave followed by dippinginto water. Squares were formed by effecting 11 cuts respectively inlength and width at about 1.5 mm intervals on the film of the coatedplate by using a knife. An adhesive cellophane tape having a width of 24mm was adhered to the squares, followed by strongly peeling the tape toevaluate the adhesion properties of the squares according to the above(*5) criteria.

[0061] (*7) Corrosion resistance: The aqueous coating compositionsobtained Examples and Comparative Examples were applied to the insidesurface of steel two-piece cans having capacity of 250 cc by a hot airspray coating to form coating films 15 μm (when dried), followed bybaking at 215° C. for 60 seconds to obtain the coated two-piece canbodies. 10 % pineapple juice was heated at 98° C. and packed into thecoated two-piece can, followed by hermetically sealing the can. Afterstorage at 37° C. for 6 months, the can was opened and the degree ofcorrosion of the inside of, the can was visually evaluated according tothe following criteria.

[0062] A: No change

[0063] B: Slight corrosion

[0064] C: Remarkable corrosion

[0065] (*8) Flavor-retaining ability: Tap water (250 cc) treated withactivated carbon was packed into the coated two-piece can obtainedsimilarly as above (*7). The can was hermetically sealed and treated forsterilization at 100° C. for 30 minutes. After storage at 37° C. for 6months, the liquid in the can was tested for flavor-retaining abilityand evaluated according to the following criteria.

[0066] A: No change in flavor

[0067] B: Slight change in flavor

[0068] C: Remarkable change in flavor TABLE 2 Example ComparativeExample 1 2 3 4 1 2 3 4 Polyester resin kind A-1 A-2 A-1 A-3 A-1 A-1 A-4A-5 Amount 100 100 100 100 100 100 100 100 Cyclohexanone 50 50 30 50 5050 Solvesso 1500 20 Butylcellosolve 50 Propylene glycol 50 monomethylether Novolac epoxy resin 10 10 10 10 10 10 10 10 Dimethylethanol-amine5.0 7.2 5.0 5.5 5.0 5.0 4.4 5.0 Neutralization equivalent 0.7 0.7 0.70.7 0.7 0.7 0.7 0.7 Deionized water 275 273 275 275 275 275 276 275Total amount 440 440 440 440 440 440 440 440 Emulsion appearance GoodGood Good Good Good — Good Good Storage stability Good Good Good Good *— Good Good (40° C. × 1 month)

[0069] TABLE 3 Example Comparative Example 1 2 3 4 1 3 4 Film surfacecondition ∘ ∘ ∘ ∘ ∘ ∘ ∘ Gel fraction ratio 85 87 84 86 83 84 83Processability ∘ ∘ ∘ ∘ ∘ X ∘ Water resistance ∘ ∘ ∘ ∘ ∘ ∘ ▴ Adhesion ∘ ∘∘ ∘ ∘ ∘ ∘ Adhesion after water resistance ∘ ∘ ∘ ∘ ∘ ∘ ▴ test Corrosionresistance ∘ ∘ ∘ ∘ ∘ ∘ ∘ Flavor-retaining ability ∘ ∘ ∘ ∘ ∘ ∘ X

Production of Aqueous Coating Composition Example 5

[0070] The flask was charged with the polyester resin (A-1) obtained inPreparation Example 1 and cooled until 150° C. 50 parts of cyclohexanoneper 100 parts of the resin was added and the mixture was cooled until100° C. When the temperature was 100° C., 10 parts of “EHPE-3150,(manufactured by Daicel Chemical Industries, Ltd., alicyclic epoxyresin) as a curing agent was added and the mixture was dissolved inuniformity. Subsequently, 5 parts of dimethylethanolamine was added, andthe mixture was neutralized and diluted with 275 parts of deionizedwater to obtain a transparent or opaque white water dispersion having asolid content of 25 %. The water dispersion had good emulsion appearancewithout precipitating after the storage at 40° C. for one month.

[0071] The resulting water dispersion was mixed with 1 part of cholinechloride as a curing catalyst and 10 parts of isopropanol a surfactant,followed by stirring to obtain an aqueous coating composition.

[0072] Example 6, 7 and Comparative Example 5 to 8

[0073] The general procedure of Example 5 was repeated, except that thecompositions shown in Table 4 were used, to obtain aqueous coatingcompositions having a solid content of 25 %. But, in each composition ofComparative Example 6 and 7, water dispersion could not be performed,and therefore the aqueous coating composition was not obtained.

Performance Test

[0074] The aqueous coating compositions obtained Example 5 to 7 andComparative Example 5 to 8 were applied to tin plates with a bar coaterto form coating films 10 μm (when dried), followed by baking at 230° C.for 10 minutes. Each coating film was subjected to the followingperformance tests. The result is summarized in Table 5.

[0075] (*1) Film surface condition: The surface of the film was visuallyevaluated according to the following criteria.

[0076] A: Excellent smoothness with no foaming over the entire film

[0077] B: Slight unevenness with small foam all over the film

[0078] C: Slight unevenness with large foam all over the film

[0079] (*2) Adhesion: Squares were formed by effecting 11 cutsrespectively in length and width at about 1 mm intervals on a film of acoated plate by using a knife according to JIS K-5400. An adhesivecellophane tape was adhered to the squares, followed by strongly peelingthe tape to measure a number of remaining squares per 100 squares.

[0080] (*3) Pencil hardness: A pencil hardness of the coated plate wasmeasured by pencil scratch examination of JISK-5400.

[0081] (*4) Bending resistance: A coated plate was placed at 20° C.atmosphere and bent in a right angle in two or three seconds in suchaway that the film was outside. A peeling condition of the bent portionof the coated plate was evaluated according to the following criteria.

[0082] A: No change

[0083] B: Peeling and cracking was found TABLE 4 Example ComparativeExample 5 6 7 5 6 7 8 Polyester resin kind A-1 A-6 A-1 A-1 A-1 A-7 A-8Amount 100 100 100 100 100 100 100 Cyclohexanone 50 50 30 50 50 Solvesso1500 20 Butylcellosolve 50 Propylene glycol 50 monomethyl ether Novolacepoxy resin 10 10 10 10 10 10 10 Dimethylethanol-amine 5.0 7.2 5.0 5.05.0 4.4 5.0 Neutralization equivalent 0.7 0.7 0.7 0.7 0.7 0.7 0.7Deionized water 275 273 275 275 275 276 275 Total amount 440 440 440 440440 440 440 Emulsion appearance Good Good Good Good — — Good Storagestability Good Good Good * — — Good (40° C. × 1 month)

[0084] TABLE 5 Comparative Example Example 5 6 7 5 8 Film surfacecondition ◯ ◯ ◯ ◯ ◯ Adhesion 100/100 100/100 100/100 100/100 100/100Pencil hardness 2H H 2H 2H B Bending resistance ◯ ◯ ◯ ◯ ◯

I Claim:
 1. A polyester-based aqueous coating composition comprising amixture of carboxyl-functional polyester resin (A) which is acondensation product of at least one polyalcohol of which ethyleneglycol comprises at least about 60 mol % based on the total polyalcoholcomponent and at least one polybasic acid of which polyvalent aromaticcarboxylic acid comprises at least about 80 mol % based on the totalpolybasic acid component, and which has a number average molecularweight of about from 1,000 to 20,000 and an acid value of about from 10to 170 mgKOH/g; a water-insoluble epoxy resin (B); and hydrophobicsolvent (C); the mixture being neutralized with neutralizer (D) anddispersed or dissolved into water.
 2. An aqueous coating compositionaccording to claim 1 wherein the carboxyl-functional polyester resin (A)has a hydroxyl value of about 10 mgKOH/g or less.
 3. An aqueous coatingcomposition according to claim 1 wherein the water-insoluble epoxy resin(B) consists essentially of Novolac based epoxy resin.
 4. An aqueouscoating composition according to claim 1 wherein the water-insolubleepoxy resin (B) is selected from alicyclic epoxy-functional resins andglycidyl-functional acrylic resins.
 5. An aqueous coating compositionaccording to claim 1 wherein an equivalent ratio of the carboxyl groupof the polyester resin (A) and the epoxy group of the epoxy resin (B) isabout from 1/0.3 to 1/1.5.
 6. An aqueous coating composition accordingto claim 1 wherein the hydrophobic solvent (C) consists essentially ofcyclohexanone.
 7. An aqueous coating composition according to claim 1comprising about from 1 to 200 parts by weight of the hydrophobicsolvent (C) per 100 parts by weight of the total resin solids.
 8. Anaqueous coating composition according to claim 1 comprising about from0.05 to 3 parts by weight of a curing catalyst per 100 parts by weightof total solids of the polyester resin (A) and the epoxy resin (B).